1. Field of the Invention
The present invention relates to a radar system, and, more particularly, to a coherent radar system utilizing a delay line in order to serve as system local oscillator and produce pulse-to-pulse coherency.
2. Description of Related Art
A radar system in its primary major elements may include a radio frequency oscillator which is controlled by a pulse modulator, or pulser, so as to produce periodic pulses of high power but of relatively short duration. These pulses are applied to a highly directional antenna, typically adapted for rotation, and the antenna is controlled to transmit the pulses toward a target or towards a region in which a target is sought. A receiver, which may be interconnected with the same antenna as the transmitter, is controlled by a switching arrangement ("duplexer") to interconnect the antenna to the receiver during the inerval between transmitted power pulses in order to receive reflected energy from a target. By monitoring the antenna direction and timing of reflected pulse returns, location and range of a target can be obtained.
There are other radar systems for detecting targets and their ranges which rely upon the radiation of unmodulated continuous wave energy. The present invention is not concerned with these continuous wave or "CW" radar systems, but rather with pulse type systems.
One of the earliest forms of oscillators for producing radar pulses was a device called a magnetron, which has an undesirable charateristic in that the phase of each transmitted pulse is randomly related to the phase of previous pulses. The phase problem requires special adjustment and control when the radar is being used as a moving target indicator (MTI). That is, a primary use of radar, for either domestic or military applications, is to detect a moving target amid echoes obtained from adjacent fixed objects, the latter frequently being referred to as "ground clutter" or just "clutter".
More particularly, it is necessary for a radar to act as a satisfactory MTI that the phase of a transmitted pulse be made coherent with the system local oscillator (LO), and it is this coherence that we are primarily concerned with here and in connection with which the described invention achieves its primary utility and advantages.
Modern MTI radars usually achieve coherency by using a very stable master oscillator. This master oscillator is very expensive, especially for missile radar that are produced in large volume. Conventional coherent radar systems using stable oscillators have not been considered affordable for this class of weapons, and it is, therefore, highly desirable to find reduced cost alternatives.
In the case of air-to-air missile seekers, they usually use medium PRF or high PRF, both of which are ambiguous in range necessitating maintaining pulse-to-pulse coherence. The present invention can achieve pulse-to-pulse coherence by locking the phase of each transmitted pulse to the delayed replica of the previously transmitted pulse.
The early radar systems, generally operating at lower frequencies than used now, achieved coherency between the LO and the transmit pulse by locking the LO to the transmit phase and maintaining the locked phase during the listen interval. This technique, however, did not provide pulse-to-pulse coherence.
At the present time, fully satisfactory coherent radar seekers having pulse-to-pulse coherence require a relatively expensive frequency reference unit (FRU) that consists of stable oscillators which operate over wide temperature ranges. This necessity for an FRU is considered to make some missile radar seeker systems prohibitively expensive.
It has been suggested for shipboard use to detect low flying targets over the sea surface that a high resolution radar be provided with a noncoherent delay line canceller, the delay line being of the fiber optic variety. C. T. Chang, D. E. Altman, D. R. Wehner, D. J. Albares, "Noncoherent Radar Moving Target Indicator Using Fiber Optic Delay Lines," IEEE Trans. on Circuits and Systems, Vol. CAS-26, No. 12, Dec. 1979, pp. 1132-1135.